JP 1998-246206A discloses a control valve apparatus including a pair of actuator ports, a spool, and a lock valve mechanism. The pair of actuator ports communicates with an actuator. The spool controls communication between the pair of actuator ports and each of a hydraulic pump and a tank. The lock valve mechanism is provided in an oil passage for one of the actuator ports, allows circulation of supply oil to the actuator, and allows circulation of return oil from the actuator only when an operation signal has been given. The lock valve mechanism includes a seat valve and a pilot valve portion. The seat valve opens and closes the oil passage. The pilot valve portion selectively brings a back pressure chamber of the seat valve into communication with an outlet side of the seat valve or the tank. The back pressure chamber of the seat valve communicates with the tank via a drain port formed in a valve block.